Conventionally used seals in removable closure means for sealing containers, are substantially gas-permeable and therefore ineffective, over a long period of time, greater than six months, to negate the effect of deleterious gases which diffuse through the seals and degrade contents which are sensitive to such gases. Sealing elements or closure liners for such closures are typically molded closures which include twist crowns, crown corks, stoppers, septums for syringe vials, screw caps for bottles jars and the like but may also be gaskets; many of these are made by in-shell or out-shell molding and gaskets may also be cast in-situ.
For in-shell molding, most commonly used, granules of blend are fed into an extruder and a rotating blade cuts the extrudate into a pellet which is dropped into the bottle cap or other closure. The extrudate does not adhere to the blade and the pellet, because of its low “tack”, is easily positioned in the cap. A “tacky” blend is one which, when extruded, adheres to the blade. In out-shell molding, the pellet is formed outside of the closure, on a “puck”; the pellet is then positioned in the closure and molded into its final shape. After cooling and hardening of the thermoplastic polymer compound, the shaped seal forming the cap liner is soft enough to be deformable so as to seal the bottle rim when the cap is tightened onto the mouth of the bottle.
Oxygen-containing gases, and molecular oxygen and carbon dioxide in particular, are known to affect the storage life of a fruit juice or drug adversely, despite being tightly sealed in a glass container with a conventional TPE seal. For example, permeation of oxygen through a seal is detrimental to fresh fruit juice even when the containers are stored under atmospheric pressure. The permeation rate increases with pressure. An inert gas blanket which may be sealed in a container at a pressure up to about 2 atm (atmospheres or bar) may be lost through the seal in a tightly secured cap over a period less than six months.
The product of this invention provides a soft seal having a hardness in the range from Shore A 30 to 90, which is essentially gas-impermeable so long as the gas exerts a pressure of less than about 3 atm (or bar). Though the pressure does not affect permeability which is the permeation rate normalized for a 1 mil (2.54 μm) thickness and 1 atm, the permeation rate at 3 atm is so high that it requires an uneconomically thick seal to provide the desired barrier against oxygen permeation. The thermoplastic blend of this invention provides an extrudable, injection-moldable or blow-moldable shaped article of arbitrary shape, most commonly a laminar sheet, consisting essentially of a blend of synthetic rubber and polyisobutene plasticizer; when formed into a seal having specified hardness and/or specified melt viscosity, the seal may be removably secured to the mouth of a container to seal its contents against a damaging concentration of oxygen permeating through the seal.
Sealing a container against leakage of a liquid under relatively low pressure, in the range from about 1 to 3 atm (or bar), either into or out of the container, is a relatively trivial problem compared to providing an essentially gas-tight and penetrant-impermeable seal under the same pressure. Some medical products, such as injectable drugs which are sensitive to reaction with a gas are typically packaged in essentially gas-impermeable bottles and sealed with an elastomeric seal such as a stopper or plug, without a cooperating closure means. For example, the seal, by itself, may be forcibly conformed to the mouth of a container; or the seal may be otherwise held in sealing relationship with the mouth of the container. Permeation of penetrant gas through the body of the container itself, whether a package, jar, bottle or vial containing medication, is easily negated by making the body from a gas-impermeable inorganic material such as glass; or from an essentially gas-impermeable engineering thermoplastic such as a polyamide, ethylene vinyl alcohol, polyvinyl alcohol, polycarbonate, polyacetal, ABS resin, polybutylene terephthalate, polysulfone, aromatic polyester, polyphenylene oxide blend, and the like. However, the leakage of concern in this instance is not that which occurs from around the periphery of a purportedly gas-tight seal, but by gas permeation, that is, by movement of the penetrant into the polymer, diffusion of the penetrant through the polymer, and, desorption and evaporation of the penetrant from the surface of the polymer.
All containers are configured so as to be sealed to minimize the leakage of gas which then becomes trapped in contact with the gas-sensitive product held in the container. It is well known that an essentially gas-impermeable adequately soft and thin TPE cannot now be injection-molded in conventional injection-molding machines economically. Known TPEs which have oxygen-permeability less than 40,000 cc.(2.54 μm)/m2.day.atm, have a hardness greater than Shore A 90 and are too hard to provide easily usable seals. A usable TPE product is defined as a rubbery synthetic resinous material required to have a hardness in the range from Shore A 30 to 90 and lower than the aforestated oxygen-permeability.
The TPE is chosen from (i) a block copolymer of a vinylaromatic compound and a conjugated diene, which optionally, is at least partially hydrogenated, and (ii) a thermoplastic vulcanizate (referred to as a “TPV”). The block copolymer (i) may be a diblock, triblock, tetrablock or star block copolymer, but is typically a triblock of either styrene-butadiene-styrene, or styrene-isoprene-styrene. The TPE (i) is therefore referred to herein, for convenience and brevity, as a “SBS” copolymer. Either (i) or (ii) may be too soft or too hard, before it is plasticized, to be used as a desirable elastomeric product, usable as a removable sealing element in hardness range from Shore 30 A to 90. In either case the starting TPE has an oxygen-permeability greater than 40,000 cc.(2.54 μm)/m2.day.atm at 23° C. which is unsatisfactory. When either (i) or (ii) is too hard, it may be melt-blended with an unreactive polymono(lower)olefin, the olefin having from 2 to 4 carbon atoms, preferably with more than 5% by weight of the TPE. A TPE with unsatisfactory oxygen-permeability is converted to one in which the oxygen-permeability is satisfactory for use as a sealing element when it is plasticized with “liquid polyisobutene”, as disclosed herein.
The physical properties of the melt-blended SBS may be modified with a “plastic” polymer, typically an α-β monoolefinically unsaturated hydrocarbon polymer, and in addition, optionally, a rubber, each of which is a non-reactant relative to the other and to the TPE; typically, the plastic and rubber are non-crosslinkable with sulfur, peroxides and other conventional crosslinking agents. TPE seals are typically essentially inert, that is, unreactive with either the contents of the container or with inorganic or organic liquids or gases in the environment.
Starting TPEs are known TPVs and SBSs. Most preferred is a SBS which is a tri-block copolymer having either a poly(diene) or a poly(monoolefin) mid-block (“M-block”) and vinylaromatic, preferably polystyrene end-blocks (“S-blocks”). There is need for a practical, readily deformable, sufficiently oxygen-impermeable seal which would provide an effective barrier against permeation of oxygen through a cross-section of material less than 10 mm thick, preferably less than 5 mm thick, over a long period of time in the range from about 1 to 10 years.
It is self-evident that a conventional TPE seal in a sufficiently very large thickness (cross-section) will be essentially gas-impermeable, but it is equally self-evident that it is impractical and uneconomical to provide a seal or a liner in such sufficiently very large thickness.
The problem is to provide an injection-moldable, soft and flexible, essentially gas-impermeable TPE seal, usable in a thickness in the range from about 0.1 mm to 10 mm, which is effective as a barrier against the permeation of a deleterious gas through the seal. An effective seal provides both, an adequately low permeation rate and also an adequately low transmission rate. The lower the permeation rate, the lower the transmission rate and the better the barrier properties. For the purpose at hand, only the oxygen permeation rate and oxygen-permeability is considered because of its particular importance. Permeation rate is measured over the actual thickness of the cross-section of polymer. Factors which affect permeation rate are temperature, relative humidity, material thickness, pressure which is usually barometric pressure, and time. Permeability is the same measurement normalized for a thickness of 2.54 μm (1 mil or 0.001 inch) and 1 atm; or, cm3.(2.54 μm)/m2.day.atm; that is, cm3 of oxygen per 2.54 μm cross-section/m2 per day.atmosphere. A TPE having sufficiently low gas-permeability will provide a solution to the problem and ensure that the contents of the container will have a desired greatly extended shelf-life relative to the shelf-life obtained with currently used TPE seals.
Polybutene, whether homo- or copolymers of isobutene, 1-butene (α-butylene) and/or 2-butene (β-butylene, whether cis- or trans-) irrespective of the ratio of the repeating units, and polymers of higher alkenes having from 5 to about 8 carbon atoms (“poly(higher)alkenes”), are typically rubbery solids. But such rubbers, by themselves have unsatisfactorily high oxygen-permeation rates. Since a thin seal of an adequately deformable block copolymer, formed of S-blocks and a M-block of a conjugated diene, or, of a mono(lower)olefin, and no harder than Shore A 90, provides an unsatisfactorily high oxygen-permeation rate, it is not surprising that a rubbery poly(higher)alkene would also provide a comparably unsatisfactory oxygen-permeation rate.
To combat the unsatisfactory baffier properties of butyl rubber, U.S. Pat. No. 5,731,053 to Kuhn teaches that the butyl rubber is to be heterogeneously blended with high density polyethylene (HDPE) or mixtures of HDPE with another polyolefin, so that areas enriched in butyl rubber alternate with areas enriched in the polyolefin. Kuhn also teaches that good barrier properties are obtained with a 50/50 blend of medium density polyethylene (MDPE) and butyl rubber but the blends fail in a headload test. He failed to note that when sufficient polyolefin is added to SBS rubber to improve permeability of the blend, the hardness of the blend is unacceptably high, typically higher than Shore A 90.
A liquid homopolymer of isobutene (isobutylene) is commercially available, and an isobutene-co-butene copolymer, in which butene is present in a minor molar proportion, may also be produced as a liquid. The homo- and copolymer are together referred to as “polyisobutene” herein. Its manufacturer teaches applications of the liquid polymer in adhesives. The text of those teachings are set forth in full below: